Leadless chip carriers are rectangular packages for mounting integrated circuit chips with respect to printed circuit boards. Developed to improve upon the presently more common DIP (dual in-line package), the leadless chip carrier replaces the outwardly protruding leads of the DIP with electrically conductive pads distributed about the periphery of the carrier's bottom surface. The leadless chip carrier thus offers the same number of electrical contacts using 70 percent less substrate surface area for a more effective use of circuit board surface area. The chip carrier pads also form shorter, more reliable conductive paths with significantly reduced inductance and capacitance, allowing the chip carrier to handle higher frequencies. The shorter paths of course reduce signal transmission time.
Leadless chip carriers can be constructed of a plastic, or of a ceramic such as aluminum oxide. A particuar advantage of the ceramic leadless chip carrier, both with respect to dual in-line packages and the plastic leadless chip carriers, is the ability to hermetically seal the IC chip within the carrier, thus to provide a chip usable in environments that require an absolute moisture seal.
Despite these advantages, the leadless ceramic chip carrier is not well adapted for mounting directly to a conventional circuit board in an environment of temperature extremes This is due to the difference in thermal expansion coefficient between the ceramic carrier and printed circuit board, commonly referred to as thermal mismatch. When the printed circuit board and ceramic chip carriers mounted thereto experience temperature extremes, the different rates at which they expand or contract causes stress concentrations at the interfacing solder joints. Repeated expansions and contractions (cycles) cause cracking at the joints and increases electrical resistance to unacceptable levels.
There have been a number of attempts to solve the thermal mismatch problem. One approach involves special sockets into which the leadless chip carrier is inserted, the socket in turn being mounted to the printed circuit board. Aside from the added expense, these sockets eliminate most of the advantages of the carriers in size and shorter conductive paths. Alternatively, materials more thermally compatible with the ceramic chip carrier have been sought for printed circuit boards. The expense of this approach has been prohibitive. Yet another approach is to build up "pedestals" of conductive material such as copper on the circuit board contacts where the carrier pads are to be positioned. While this does increase flexibility, it weakens the connection and is labor intensive.
Another problem associated with leadless chip carriers is difficulty in their manufacturing and testing, since the carrier structure is not well adapted for automatic handling. Testing typically is performed using specially designed sockets which are expensive and increase handling since each carrier must be manually inserted, tested, then manually removed. Burn-in, if required, calls for special sockets in addition to the ordinary electrical testing sockets, multiplying the handling difficulties.
Therefore, it is an object of this invention to provide an inexpensive means for mounting a ceramic chip carrier to a conventional printed circuit board to withstand repeated thermal cycles over a wide temperature range. It is a further object of the invention to provide a simple, inexpensive integrated circuit package suitable for direct mounting to a conventional circuit board for use in a demanding environment involving temperature extremes. It is yet another object of this invention to provide a process for facilitating automatic manufacture and testing of integrated circuit packages including leadless ceramic chip carriers.